The Federal Department of Housing and Urban Development (HUD) and its local agencies require installation of security screens for various housing projects throughout the country. The screens are typically installed over windows on the exterior of the first floor to protect the glass from vandalism and to prevent forced entry by intruders.
In order to ensure the effectiveness of the security screens, HUD, in conjunction with the Screen Manufacturer's Association, set minimum specifications (the "SMA Specifications") for, among other things, the ability of security screens to withstand direct impacts, sustained loads and attempts at entry. These specifications and the related definitions have been published as the American National Standard Specifications for Metal Protection Screens (ANSI/SMA 6001-1900) and are herein incorporated by reference.
The most rigorous of the SMA Specifications are for so-called "heavy" screens which are intended to prevent glass breakage and/or to deter forced entry through openings which are easily accessible. The "heavy" standards require in part that, during an impact by an 11 inch ball generating at least 100 lbf-ft (136 N-m) of force, a screen cloth must not stretch more than 3 inches. This distance is measured from the original plane of the screen cloth to the point of maximum displacement and applies to both temporary and permanent deformation. This procedure tests the strength and rigidity of both the screen cloth and the frame. The purpose of the limitation is to prevent breakage of the underlying glass due to objects thrown at the screens and to inhibit forced entry.
Typically, security screens are made with two frames. One frame, called the subframe, is fastened directly to the window opening, usually with bolts or screws. This subframe anchors the screen and prevents it from being removed. Another frame, the main frame, holds the wire cloth and is attached to the subframe with hinges. There is also a lock that is used to secure the main frame to the subframe when closed.
One method to increase a screen's ability to withstand an impact force is to increase the dimensions of the main frame. This is undesirable however because of the increased costs, weight, and line-of-sight of the frame.
Another method comprises springs connecting the screen cloth to the main frame. While a screen of this design can withstand a greater impulse without deforming plastically or tearing, the design does not limit the peak elastic deformation of the screen as required by the SMA Standards.
Another method to strengthen a screen is to provide a rigid support connecting opposite sides of the frame across the screen cloth. However, the drawback of this design is that the support blocks vision and light.
Therefor, what is desired is a means to decrease the permanent and temporary displacement of a screen cloth and its main frame when subject to a force on the screen cloth without increasing the dimensions of the screen cloth or the main frame and without decreasing the amount of area through which light may pass.